1. Field of the Invention
This invention relates to a differential encoding method and a device for carrying out the method and more particularly to an adaptive type differential encoding method and a device which compresses color image signals.
2. Description of the Related Art
Transmission systems for transmission of digital image information in a compressed state have recently come to be actively developed for TV conference systems and full-color transmission of still pictures. Among known methods for such transmission, a differential pulse code modulation (hereinafter referred to as DPCM) method has attracted attention and considered to be effective especially for digital transmission of image information through a narrow band transmission route. In the DPCM method of the prior art, color images are expressed by colorimetric systems of YIQ, Y.multidot.R-Y.multidot.B-Y, CIELAB, CIELUB, etc.; and, for each of the three parameters of colorimetric systems, a differential value is obtained and quantized.
These colorimetric systems, however, do not always present uniform spaces in terms of human visual perception. A color difference visually perceptible by man for a change taking place among colors of the same norm greatly varies with their positions within the colorimetric system. In the conventional DPCM method, therefore, data has not been compressed in a manner apposite to the characteristic of visual sensation of man.